How do you hide the ends of a plane?
When making landscapes  things to consider 1ST - Very Important - Your Camera angle and your camera view. How much of the Landscape is going to be visible thru your camera only make and entertain the Landscape till there. 2nd - Each and every 3d landscape you see are overlaid / Overlapped onto one another so that there boundaries just disappear somewhere while overlapping and then further covered with additional rock/plant/grass/debris etc details.
Consider this your landscape and think about what I mentioned above.